CCD Imager with improved low light level response

ABSTRACT

At low light levels, the floating diffusion output stage of a CCD imager has the peaks of the reset pulses applied to its reset gate reduced. This introduces spatial integration in the direction of line scan that improves the signal-to-noise ratio of the video samples supplied as imager output.

The present invention relates to CCD imagers using floating diffusionoutput stages and, more particularly, to resetting the floatingdiffusion output stages so as to obtain improved response to low lightlevels.

When operated conventionally, the CCD floating diffusion output iscompletely reset on each clock cycle to a fixed potential established bythe reset drain. The floating diffusion acts a capacitor which isinitially charged to the reset drain voltage and subsequently dischargedby the incoming charge packet by an amount proportional to the packetsize. Then, before the next charge packet is dumped onto the floatingdiffusion, the floating diffusion is again completely reset to the resetdrain potential. Owing to complete reset, there is no memory of previouscharge packets. Therefore, the voltage swing obtained at the floatingdiffusion can only be as large as Q/C, where C is the capacitance at thefloating diffusion node and Q is the signal charge contained in apacket. For the case of an imager with many resolution elements, thearea of each element is made smaller. This is because of yield and lensformat considerations. Therefore, increased resolution results indecreased size of the charge packets associated with individual pixels.The noise voltage against which the signal voltage generated by thesereduced-size packets is to be compared, in determining signal-to-noseratio, comprises as its two primary components the reset noise and thenoise of the video amplifier following the floating diffusion. The resetnoise at 300 Kelvin is about 400 (C_(fd))1/2 electrons where C_(fd) isthe capacitance of the floating diffusion. Signal-to-noise ratios arequite severely degraded at low light levels as image resolution isincreased.

The present invention is directed to trading off image resolution forimproved signal-to-noise performance as light levels are reduced.Spatial integration of the image is used to increase imager sensitivityat reduced light levels, increasing the imager output signal withoutappreciably increasing attendant noise levels. To achieve increasedspatial integration of the image at lower light levels, as the image isdescribed in video signal form, the reset of the floating diffusion ismodified so that the portion of the charge that can be transferred fromunder the floating diffusion upon application of a reset pulse isreduced in inverse relation to the level of that charge.

In the drawing:

FIG. 1 is a schematic diagram, partially in block form, of a CCD imagersystem constructed and operated according to the principles of theinvention;

FIG. 2 is a diagram of floating diffusion output stage response whenconventionally reset and when reset according to the invention;

FIG. 3 is a graph of signal-to-noise ratios for the two modes ofoperation; and

FIGS. 4 and 5 are schematic diagrams, partially in block form, ofalternative types of imager system constructed and operated according tothe principles of the invention.

FIG. 1 shows a light image 5 being conducted by optics 7 to a CCD imager10. CCD imager 10 is shown in block, with particulars of its floatingdiffusion output stage in full circuit schematic within the block. Thisfull circuit schematic includes a section 11 of the end of theburied-channel output register with silicon bulk 12 assumed to bep-type, with an output gate 13 to which a direct voltage V_(DC)establishing a barrier height is applied, with a floating diffusionregion 14 connected to the gate of a source-follower field effecttransistor (FET) 15, with a reset gate 16 connected to an imagerterminal 17, and with a drain region 18 to which a reset drain voltageV_(RD) is applied. V_(RD) is a direct voltage, preferably one that canbe adjusted to control the minimum imager output signal level at whichfull reset of floating diffusion 14 takes place. The CCD imager outputsignal terminal 19, to which the source of source-follower FET 15connects, supplies pixel samples of video signal to a filter 20. Filter20 conventionally includes some low-pass frequency-domain filteringsample-and-hold circuitry sampling at pixel scan rate. Filter 20 mayinclude a dc blocking capacitor. In any case filter 20 responds to thepixel samples to provide a continuous video signal.

In the FIG. 1 apparatus this continuous video signal is applied to anautomatically gain-controlled video amplifier 21, which is used toconvert a video signal with changing brightness to a video signal ofmore constant brightness. To this end the output signal from AGC'd videoamplifier 21 is applied to an AGC detector 22 that detects thebrightness of amplifier 21 output signal to develop an automatic gaincontrol (AGC) signal, assumed to be positive, which is applied viacontrol line 21a to control the gain of amplifier 21 by degenerativefeedback. AGC detector 22 may be of a type detecting average brightness,a type detecting peak brightness, or a type detecting a combination ofboth average and peak brightness. The range of AGC afforded by this loopis limited so amplifier 21 can gain control over a range of differingbrightnesses of, say, twenty-six to forty dB. The AGC range shouldinclude the levels of video input to amplifier 21 where signal-to-noiseis acceptably high without having to accept reduction of the spatialresolution of the image described by the video signal as finallyprocessed.

In the range of operation just described, the timing generator 23, whichsupplies via bus 24 the various clocking and timing signals required foroperating CCD imager 10, supplies positive-going reset pulses at pixelclock rate via a resistor 25 and imager terminal 17 to the reset gate16. PNP clamp transistor 26 is non-conductive owing to AGC voltagesbeing arranged to be more positive than the positive peak of the resetpulses, so the peak value of these reset pulses is unaffected in theirpassage through resistor 25 for this range of operation. The peak valueis sufficiently positive that the entirety of each charge packettransferred under floating diffusion 14 at pixel sample rate istransferred to the drain region 18 during the time the subsequent resetpulse applied to reset gate 16 causes field effect transistor conductionbetween floating diffusion 14 as initial source and drain region 18.

As the AGC voltage falls below the value for which AGC'd video amplifier21 begins to exhibit gain reduction, so that amplifier 21 gain is at itsmaximum, clamp transistor 26 begins to exert clamping on the peak valueof the reset pulses, so that charge transfer from the floating diffusion14 to drain region 18 during the duration of the reset pulse is slowed.That is, the conduction channel between floating diffusion 14 and drainregion 18 induced under reset gate 16 upon application of reset pulsetends to be more resistive as the reset pulse peak is made lesspositive, and the RC time constant associated with transfer of charge todrain region 18 becomes longer. This means that not all the charge in acharge packet transferred to floating diffusion 14 need be transferredfrom it on the subsequent reset pulse. The portion of the chargetransferred will be related to the size of the charge packet. Largercharge packets tend to bias more negative the virtual source that isprovided by floating diffusion 14, in the n-channelfield-effect-transistor action with channel between floating diffusion14 and drain region 18 which occurs when reset gate 16 is pulsed. Thisforward bias increases the transconductance of the transistor, loweringits channel resistance and thus decreasing the RC time constantassociated with transfer of charge from the well under floatingdiffusion 14. There is thus an introduction of spatial integration inthe direction of line scan in the video signal from imager 10, and theamount of integration is most pronounced for small quantity of charge onthe floating diffusion 14.

Reset operation wherein all the charge on the floating diffusion 14 (ora similar floating gate) that connects to the electrometer FET 15 gateelectrode is drained away to the reset drain region 18 during resetting,as is conventionally done, will be referred to as "hard reset". Resetoperation wherein a portion of the charge on the floating element isleft behind following each resetting, in accordance with the inventionand as described in the preceding paragraph, will be referred to as"soft reset".

FIG. 2 graphs the difference in the voltage responses to a white objectin the image scan line (shown at top of figure) of a CCD imager havingconventional resetting of its floating diffusion output stage and of aCCD imager having lessened or "soft" resetting of its floating diffusionoutput stage in accordance with the invention. The former response,shown at middle of figure, and the latter response, shown at bottom offigure, are at output signal terminal 19 of imager 10. Note theincreased amplitude of the latter "soft" reset response owing to signalsamples being spatially integrated. The spatial integration of the imagetransitions tends to introduce low-pass filtering that delaystransitions in the video signal. This is acceptable in less criticalapplications, where the video output from AGC'd video amplifier 21 maybe used directly, without further processing.

In more critical applications, amplifier 21 video output can be appliedto transient peaking circuitry 27, which will, except for time delay,correct the signal for low brightness level, and to time delay circuitry28, which will introduce corresponding delay into video signals of allbrightness levels. A cross-fader 29 can then cross-fade between peakingcircuitry 27 output video signal at low brightness levels and delaycircuit 28 video output signal at high brightness levels to provide afully processed video output signal. The cross-fading is shown beingcontrolled by AGC signal supplied to it via control line 29a fromdetector 22.

The peaking of transitions at low brightness levels makes the noisespectrum triangular, increasing with frequency increase. The humanobserver of an image recreated from the video signal findshigher-spatial-frequency noise less objectionable thanlower-spatial-frequency noise, so a triangular noise spectrum ispreferable to a flat noise spectrum for given signal-to-noise ratio.

FIG. 3 illustrates the differences in signal-to-noise ratio obtainableusing conventional "hard" reset and the newly invented "soft" reset ofthe floating diffusion output stage. The experimental data on which thegraphic information is based were obtained using a CCD imager having a540×512 array of pixels in its image register, into whichvertical-bar-pattern images were projected. The peak level of "soft"reset pulses applied to the reset gate was fixed, and tests were madefor 100% of full well and 5% of full well. A nine dB improvement at lowfrequencies is observed for full-well operation. Because of the adaptivetransconductance effect in the field effect transistor action of reset,a larger fifteen dB improvement is available at 5% of full welloperation.

FIG. 4 illustrates another way in which the invention may be practiced,which does not clamp reset pulse level responsive to an AGC voltage. Thevoltage V_(RD) rather than being applied directly to the drain region 18is applied instead to terminal 30 of CCD imager 10 through a currentsensing device, such as the input circuit of a current amplifier 31which provides drain region 18 a low-resistance path to reset drainvoltage V_(RD). Current amplifier 31 may, for example, be anintegrated-circuit operational amplifier of the so-called Norton type.The peak of the reset pulse is reduced in inverse relationship to thereset drain current to the input circuit of current amplifier 31, byflowing the output current of amplifier 31 through resistor 25.Capacitor 32 parallels resistor 25 to filter off video signalvariations, causing the drop across resistor 25 to be related to averagebrightness. Variations can be made in the circuitry to relate the dropacross resistor 25 to peak brightness rather than average brightness.The output of filter 20 may be gain controlled and processed similarlyto the way shown in FIG. 1, if desired.

FIG. 5 shows another way in which the invention may be practiced inwhich the reset pulses applied from timing generator to reset gate 16via terminal 17 do not vary as a function of light level. Instead theregulation of the degree of reset is effected by varying the voltageapplied to the reset drain 18 via terminal 30. This is done in FIG. 5 byreducing the voltage from a value of V_(DR) supplied from an output ofAGC detector 22 at high light level to an appropriately less positivevoltage at lower light level. This results in a shift in the directpotential level of video output signal supplied at terminal 19 of CCDimager 10, but this is of no substantial consequence where the videosignal is a-c coupled anyway--i.e. by inclusion of d-c blockingcapacitor 33 in the input to filter 20.

A number of other variations on the apparatuses described above whichuse the principles of the present invention will readily occur to oneskilled in television electronics design, and the scope of the ensuingclaims should be construed broadly to include such variations withintheir scope where possible.

What is claimed is:
 1. In the method of operating a CCD imager of thetype having a floating diffusion output stage, said method comprisingthe steps of:periodically introducing charge packets descriptive ofimage elements under the floating diffusion; sensing the potential ofthe floating diffusion following the introduction of each charge packet;and providing, during reset intervals following each sensing ofpotential and preceding the next introduction of a charge packet, aconductance path for transfer of charge stored under the floatingdiffusion to a reset drain-the improvement for providing a video signalwith improved signal-to-noise ratio responsive to low light input levelsto the imager, which improvement in said method comprises the step of:reducing the conductance of the path provided during each reset intervalfor transfer of charge stored under the floating diffusion to the resetdrain to such value as to slow the transfer of the charge stored underthe floating diffusion to the reset drain during each reset interval ininverse relation to the level of the charge stored under the floatingdiffusion, whereby there is integration of the charge packets formed inresponse to low light levels to increase the level of the signalcomponent of sensed potential respective to the noise component thereof.2. Apparatus for implementing the method of claim 1 comprising:a CCDimager having a floating diffusion output stage comprising in additionto a floating diffusion, a reset drain, a reset gate, a field effecttransistor amplifier with a gate connected to the floating diffusion anda source-to-drain circuit from which CCD imager output signal issupplied; detector means for detecting the current flow in said resetdrain; and reset pulse supplying means for supplying reset pulses tosaid reset drain having a peak level controlled in response to theoutput of said detector means.
 3. Apparatus for implementing the methodof claim 1, comprising:a CCD imager having a floating diffusion outputstage comprising in addition to a floating diffusion, a reset drain, areset gate, a field effect transistor amplifier with gate connected tothe floating diffusion, and a source-to-drain circuit from which CCDimager output signal is supplied; means for converting the CCD imageroutput signal to continuous video signal; a gain-controlled videoamplifier having a video signal input circuit to which said continuousvideo signal is applied, having a gain control signal input circuit, andhaving a video output signal circuit; an AGC detector having an inputcircuit connected to the output circuit of said gain-controlled videoamplifier and having an output circuit connected to the gain controlsignal input circuit of said gain-controlled video amplifier, forcompleting a feedback loop generating an automatic gain control signal,responsive to a range of higher light levels to said CCD imager forregulating the brightness of video response from said video outputsignal circuit, said higher light levels being those having an adequatesignal-to-noise ratio; means for applying a direct reset drain voltageto said reset drain; and means for supplying reset pulses to said resetgate, which pulses are of a peak amplitude regulatable by said automaticgain control signal generated responsive to a range of lower lightlevels to said CCD imager, said lower light levels including those forwhich improved signal-to-noise ratio is provided.
 4. Apparatus as setforth in claim 3 further including:means responsive to said automaticgain control signal being generated responsive to said range of lowerlight levels to said CCD imager, for peaking signal transitions in videosignal taken from said gain controlled video amplifier output signalcircuit.
 5. Apparatus for implementing the method of claim 1,comprising:a CCD imager having a floating diffusion output stagecomprising in addition to a floating diffusion, a reset drain, a resetgate, a field effect transistor amplifier with a gate connected to thefloating diffusion, and a source-to-drain circuit from which CCD imageroutput signal is supplied; means for converting the CCD imager outputsignal to a video signal; a gain-controlled video amplifier having avideo signal input circuit to which said video signal is applied, havinga gain control signal input circuit, and having a video output signalcircuit; an AGC detector having an input circuit connected to the outputcircuit of said gain-controlled video amplifier and having an outputcircuit connected to gain control signal input circuit of saidgain-controlled video amplifier, for completing a feedback loopgenerating an automatic gain control signal, responsive to a range ofhigher light levels to said CCD imager for regulating the brightness ofvideo response from said video output signal circuit; means forsupplying reset pulses of a fixed amplitude to said reset gate; andmeans for applying to said reset drain a reset drain voltage relatedover a range of light levels to said automatic gain control signal. 6.Apparatus as set forth in claim 5 further including:means responsive tosaid automatic gain control signal being generated responsive to saidrange of lower light levels to said CCD imager, for peaking signaltransitions in video signal taken from said gain controlled videoamplifier output signal circuit.